1. Field of the Invention
The invention relates to a contact element for supplying electric current to substantially board-shaped objects that are to be treated by an electrolytic process.
2. Brief Description of the Related Art
To submit objects to an electrolytic treatment, they have to be transported and placed in electric contact in order for them to be supplied with electric current while being in contact with a liquid for treatment.
Various appliances are used to serve this purpose. U.S. Pat. No. 4,767,146 for example discloses holding tongs for printed circuit boards which comprise two legs provided each with two gripping arms. On account of the force of a spring, the legs with the gripping arms are pivotal about an upper axis, the lower ends being thus pushed together. The lower ends of the gripping arms are provided with teeth. The arms of the one leg are provided with two tooth-like projections each and the arms of the other leg with one tooth-like projection each.
This contacting and holding appliance is used in conventional electrolytic tank plants in which the printed circuit boards are vertically held and dipped into the tank.
Another method consists in conveying the objects through the various processing stations in horizontal direction.
DE 25 12 762 B2 suggests using electrical contact springs for contacting objects to be chromium-plated that are fastened on work holding fixtures. For this purpose, the objects are clamped in the contact springs which are formed by two springable bars.
For some time past, employment has been made of coating lines for the electrolytic treatment of board-shaped objects, in particular for printed circuit boards, the boards being transported through said coating plants in horizontal direction. For this purpose the boards are contacted with the liquid for treatment, with a galvanizing bath or with a solution suitable for electrolytic pickling for example. In order to allow electric current to be supplied to the boards, appropriate contact elements are provided. The boards are moreover led through the plant by conveying facilities.
Furthermore, WO 97/37062 A1 describes a device for the electrochemical processing of electrically conductive areas which are insulated against each other on printed circuit boards. To make electrical contact with these areas, brushes are used, whose thin conductive fibers make electrical contact with the structured surfaces to be treated. The printed circuit boards are guided past the stationary brushes in horizontal direction and in horizontal orientation so that the tips of the brushes graze the surfaces. This device is not suited for transmitting large electric currents to the printed circuit boards. It is also difficult to find a solution for practical operation that, on one hand, provides the brushes with a long working life and that, on the other hand, protects the surfaces of the printed circuit boards against damage.
DE 36 45 319 C2 discloses an array for the electrolytic treatment of board-shaped objects such as printed circuits. In this array, the printed circuits are conveyed in horizontal direction and in horizontal orientation while being held by clamps which are used as conveying and contacting facilities. The clamps each consist of two bars which, under the pressure of the spring; are pivoted about their mutual, electrically conductive connecting or contacting point in such a manner that their lower ends are pushed against one another, thus grasping the side edge (galvanoedge) of the printed circuit boards. In practice, the width of the edge amounts to 10 to 15 mm. Each printed circuit board is typically contacted on at least one side by way of several clamps.
In practical operation, the flow of current is progressively increased in these plants in order to accelerate the electrolytic treatment of printed circuit boards. Nowadays, typical values for the flow of current already range from 20 to 80 A for every printed circuit side and clamping contact when the distance between the clamps amounts to 60 mm for example. The currents supplied to one whole printed circuit side by means of several clamps are considerably larger.
If, for the electrolytic treatment of printed circuit boards coated with copper on both sides, clamps that grasp both sides and supply electric contact are used, one such clamp carries 40 to 160 A. Such large currents are difficult to be transmitted onto the thin conductive base coating of the boards. Large currents cause the metal ions in the vicinity of the contacts to deplete in the electrolyte so that the deposited coating of metal scorches in the neighborhood of the contacting spots on the printed circuit boards (=formation of metal coatings with a granular crystal structure).
It has also been observed that the occurrence of the presented phenomena increases when a base coating of copper having a reduced coating thickness is electroplated. For various reasons it is necessary to reduce the coating thickness from hitherto 17 μm to at present 6 μm and even to 0.7 μm for example (when using certain manufacturing techniques [SBU-technique=Sequential-Build-Up], e.g., when fine bores (100 μm and less) must be drilled with laser light or in order to avoid undercut when thicker base coatings of copper are used, as well as for reasons of material saving and to optimize the electric properties of the boards.
These increasing demands placed on the modern fabrication of printed circuits are no longer to be met with the devices of the art. The base coatings of copper proved to “scorch” on the contacting spots and in the adjacent regions thereof on the surfaces of the printed circuit boards. This means that large black spots form at these places, where the copper coating is damaged or in parts even completely destroyed. These damages or destructions are in parts limited to the areas in which no strip conductor structures are intended to be formed on the printed circuit material. However, these damaged spots are often so large that the border areas of the strip conductor structure are damaged as well. In this case, the processed board has necessarily to be scrapped. What is particularly disturbing with these scorched spots is that the residues of scorching (fine oxidized copper) are in parts lying loose on the board and are distributed by the flow of the electrolyte over the surface of the board where they are incorporated in the coating on deposition of the metal. As a result, the board is rendered unsuitable for use.
For reasons of economy, the width for the galvanoedge is desired to be steadily reduced while the current density should increase and the thickness of the base layer of copper be lowered, which entails the above mentioned problems.
The basic problem of the invention is therefore to avoid the drawbacks of the devices and methods of the art and in particular to advance means for achieving a satisfactory electrolytic treatment utilizing very large currents while potential impairments of the metal coating on the surface of the board are merely tolerated in a very narrow border area and the above mentioned requirements can be met even on using very thin layers of copper on the boards.
The solution of the problems indicated herein above is given by the contact element, the contact organ and the method of supplying electric current according to the present invention.